The main contents of the thesis consist of two independent parts . one is the characterization of weak i sequences in terms of ext functors . we prove that the existence of such sequence and the length of a maximal weak / sequence can be expressed as some vanishing properties of ext functors 本文主要研究工作由兩個(gè)獨(dú)立的部分組成,其中一部分是利用ext函子來(lái)刻畫(huà)弱i序列,并且證明了這個(gè)序列的存在性,以及極大弱i序列具有同樣的長(zhǎng)度。